The present invention relates generally to liquid crystal display devices; and, more particularly, the invention relates to liquid crystal display devices of the thin-film transistor type, in which test procedures are made easier for inspection of the functional operabilities of thin-film transistors and connection failures at scan line lead lines and/or signal line leads. This invention also relates to a method of manufacturing the same.
Liquid crystal display devices are widely employed as high-precision color display devices for use in notebook computers and display monitor units. Currently available liquid crystal display devices include those of the simple matrix type, which comprise a liquid crystal panel with a liquid crystal layer interposed between a pair of substrates, on the inside surfaces of which parallel electrodes are formed in such a manner as to cross over each other, and those of the active matrix type, which comprise a liquid crystal display element (referred to also as a liquid crystal panel hereinafter) having switching elements for selection of pixels disposed on one of a pair of substrates.
Active-matrix liquid crystal display devices include a liquid crystal display device of the so-called longitudinal electric field scheme, such as the twisted nematic (TN) type (generally called a TN type active-matrix liquid crystal display device), using a liquid crystal panel having a group of pixel selection electrodes formed on a respective one of a pair of upper and lower substrates, and a liquid crystal display device of the so-called lateral electric field type (generally known as an IPS type liquid crystal display device) using a liquid crystal panel with a pixel selection electrode group formed on only one of a pair of upper and lower substrates.
Typically, in Tn type of liquid crystal panel of an active-matrix liquid crystal display device, liquid crystals are aligned to twist by 90° within an interior space formed between a pair of substrates (two substrates consisting of a first substrate (lower substrate) and second substrate (upper substrate)), and two polarization plates are multilayered on the outside surfaces of the upper and lower substrates of such liquid crystal panel, respectively, with the absorption axis directions thereof being disposed in a cross polarization or “crossed Nicol” fashion and also with the light entrance side absorption axes being in parallel or at right angles to the rubbing direction.
In the TN type of active-matrix liquid crystal display device thus arranged, incident light becomes linearly polarized light at an incidence side polarization plate upon application of no voltages. This linear polarized light travels along the twisting paths of a liquid crystal layer; and, if the penetration axis of a light output side polarization plate is identical to the azimuth angle of the linear polarized light, then all rays of the linear polarized light are permitted to pass out, resulting in establishment of a white display (what is called the “normally open mode”).
In the case of a voltage application, a unit vector's direction (director), indicating the average orientation direction of axes of the liquid crystal molecules constituting the liquid crystal layer, becomes perpendicular to the substrate surfaces, while the azimuth angle of incidence side linear polarized light remains unchanged, thus resulting in coincidence with the absorption axis of the light output side polarization plate, to thereby obtain a black display (see “Basics and Applications of Liquid Crystals,” issued by Industry Research Association, 1991).
On the other hand, in an IPS type liquid crystal display device having pixel selection electrodes and electrode leads formed on only one of a pair of substrates for permitting switching of its liquid crystal layer in a specified direction extending in parallel to the substrate surfaces through voltage application between neighboring electrodes (between a pixel electrode and counter electrode) on this substrate, polarization plates are so disposed as to provide the black display when no voltages are applied thereto (the so-called “normally closed mode”).
The IPS type liquid crystal display device's liquid crystal layer exhibits a homogeneous alignment or orientation parallel to the substrate surfaces in the initial state. Simultaneously the director of the liquid crystal layer in a plane parallel to the substrates is parallel or slightly angled relative to the electrode lead direction upon application of no voltages, causing the direction of the director of the liquid crystal layer in voltage application events to shift toward a direction perpendicular to the electrode lead direction upon application of a voltage thereto. When the liquid crystal layer's director direction is tilted toward the electrode lead direction by 45° in comparison with the director direction when no voltage are applied thereto, the liquid crystal layer, upon application of the voltage, causes the azimuth angle of polarized light to rotate 90° as in ½ wavelength plates resulting in coincidence between the light output side polarization plate's transmission axis and the polarized light's azimuth angle, thus providing a white display.
This IPS type liquid crystal display device has the characteristic feature that changes in color, phase and contrast remain low even at wide viewing angles, thus enabling achievement of wide view-field angles (see Japanese Patent Laid-Open No. 505247/1993).
A major approach to attain full color image visualization of the respective types of liquid crystal display devices stated supra is to employ a color filter scheme. This is achieved by subdividing a pixel corresponding to a single dot of color display into three portions and disposing color filters of the three primary colors, e.g. red (R), green (G) and blue (B), at such unit pixels, respectively.
Although the present invention is applicable to the various types of liquid crystal display devices stated above, its outline will be explained below with reference to a TN type active-matrix liquid crystal display device as an example.
As previously stated, in a liquid crystal element (liquid crystal panel) making up the TN type active-matrix liquid crystal display device (referred to simply as an active-matrix liquid crystal display device hereinafter for brevity purposes), there are formed on a liquid crystal layer side surface of one substrate of two transparent dielectric substrates, which are typically made of glass plates mutually opposed with a liquid crystal layer interposed therebetween, a group of scanning signal lines (referred to as gate lines hereinafter) extending in an “x” direction and being arranged in parallel in a “y” direction, and a group of drain lines (video signal lines) isolated from this gate line group and extending in the y direction, while being arranged in parallel in the x direction.
Each respective one of the regions surrounded by the gate line group and drain line group becomes a pixel region in which a thin-film transistor (TFT) for use as an active element (switching element) and a transparent pixel electrode are formed by way of example. When a scan signal is supplied to a gate line, the thin-film transistor turns on causing a video signal coming from a drain line to be supplied to the pixel electrode via this turned-on thin-film transistor.
Additionally, each drain line of the drain line group and each gate line of the gate line group are extended up to the periphery of the substrate to constitute external terminals, respectively, to which video drive circuits and gate scan drive circuits-namely a plurality of drive IC chips (semiconductor integrated circuits as will be referred to simply as drive ICs or ICs hereinafter) making up these circuits are connected, respectively, which are separately mounted at the substrate periphery. In other words, a plurality of tape carrier packages (TCPS) with these respective drive ICs mounted thereon are externally bonded to peripheral portions of the substrate.
However, since such a substrate is designed so that TCPs with drive ICs mounted are externally attached at the peripheral portions thereof, the occupation area of a region (generally called a “picture frame”) defined between contour lines of a display region, as formed of the substrate's cross-over regions of the gate line group and drain line group, and an outer frame of the substrate becomes larger undesirably, which is conflicts with the demand for reducing or minimizing the outside dimensions of a liquid crystal display module with the liquid crystal display element and an illuminance light source (backlight unit) and other associative optical elements integrated therein.
Thus, in order to avoid this problem, or at least minimize risks involved, i.e. to fully meet demands for high-density mountability of the liquid crystal display element and also effect downsizing of the outer size of the liquid crystal display module, the so-called flip-chip scheme or alternative chip-on glass (COG) scheme has been proposed for permitting direct mounting of drive ICs for video driving and drive ICs for scan driving on one substrate (lower substrate) without the use of any TCP components. And, the drive ICs are designed to employ the so-called FCA scheme for permitting electrodes formed on back surf aces of such drive ICs to be directly connected to electrical wiring leads formed on the substrate.
FIG. 10 is a perspective view illustrating a main part of a liquid crystal display device of the FCA mount type. This liquid crystal display device is arranged so that a liquid crystal layer is interposed between one substrate SUB1 with a matrix array of thin-film transistors formed thereon and an opposing substrate SUB2 with color filters formed thereon.
The one substrate SUB1 has one peripheral side along which scan line drive ICs (referred to as gate drivers hereafter) GDR are mounted by the FCA scheme. In addition, signal line drive circuit ICs (drain drivers) DDR are similarly mounted by the FCA scheme along another side of the substrate.
Outputs of the gate drivers GDR are connected to scan line extension leads GTM, whereas inputs thereof are connected to wiring lines of a flexible printed circuit board FPC1. Outputs of the drain drivers DDR are connected to signal line leads DTM, while their inputs are coupled to wiring lines of a flexible printed circuit board FPC2.
As shown by arrows in FIG. 10, the flexible printed circuit boards FPC1, FPC2 are arranged such that the flexible printed circuit board FPC1 can be bent in a direction BENT1 toward the back surface of substrate SUB1; and then, a curvature portion JT2 of the flexible printed circuit board FPC2 can be folded along a fold line BTL in the BENT2 direction and then folded in a direction BENT3 for accordion-like folding onto the back surface of flexible printed circuit board FPC1.
Under this condition, the flexible printed circuit board FPC2's connector CT4 can be connected to a connector, not shown, as provided on the flexible printed circuit board FPC1. An adhesive tape BAT is interposed on the inner surface of the folded portion of the flexible printed circuit board FPC2, resulting in fixture of flexible printed circuit board FPC2.
Note here that reference characters “CHG” and “CHD” designate electronics components, such as capacitors and others; ALMG, ALMD denote alignment marks; POL2 denotes a polarization plate; and AR denotes a display region.
In the liquid crystal display device with the above arrangement, probes of test/inspection equipment may be attached to extension leads of the gate lines and extension leads of the drain lines, which are extended from the thin-film transistors formed on one substrate SUB1, to thereby perform several tests for inspection of thin-film transistor characteristics and connection failures at respective electrical leads and turn-on or “lighting” tests after adhesion with the other substrate.
FIGS. 11(a) and 11(b) are diagrams of test terminals in one prior art type of liquid crystal display device, wherein FIG. 11(a) is a pictorial representation of the gate driver side, whereas FIG. 11(b) depicts the drain driver side.
In FIG. 11(a), GTM designates gate line extension leads; TPC denotes test terminals; GDR indicates gate driver mount portions (shown by dot lines); LCT denotes a laser cut line; ASCL denotes a gate line side static electricity suppression common line; GTM denotes input terminals of the gate drivers GDR.
In the manufacture of one substrate SUB1 (thin-film transistor substrate), the gate line extension leads GTM are short-circuited by the static electricity suppression common line ASCL for protection against damage to thin-film transistors and wiring leads occurring due to invasion of static electricity. Thereafter, the gate line leads GTM and individually cut along the laser cut line LCT; and then, the probes are attached to the test terminals TPC for inspection to detect connection failure or unwanted open-circuiting, while performing lighting tests upon application of more than one signal thereto.
In FIG. 11(b), DTM designates drain line extension lines; TPC denotes test terminals; DDR indicates drain driver mount portions (shown by dot lines); LCT denotes a laser cut line; ASCL denotes a drain line side static electricity suppression common line; and TTB denotes input terminals of the gate drivers GDR.
Similarly, on the drain driver side also, in the manufacture of such a substrate, the drain line extension leads DTM are short-circuited by the static electricity suppression common line ASCL for preclusion of damage due to invasion of static electricity at the thin-film transistors and wiring leads concerned. Thereafter, the drain line leads DTM are individually cut along the laser cut line LCT; and then, all the probes are attached to the test terminals TPC at a time for open-circuit inspection while performing lighting tests upon application of a signal(s) thereto.
An example of this flip-chip scheme liquid crystal display device is disclosed in Japanese Patent Laid-Open No. 122806/1996.